It is relatively common for high pressure turbine blades, as used in large combustion turbine engines, to be designed with a blade tip cap at the blade tip, the turbine blade having an airfoil-shaped hollow body with an airfoil shaped blade tip cap. The blade tip cap 2 generally has an airfoil shaped closure plate 3 and a tip rail 4 extending around the circumference of the turbine blade at the tip (see FIGS. 1 and 2). The tip rail may completely enclose the tip pocket, or may have a slot or opening, typically at the trailing edge on the concave side. In either case, it is a mandatory part of any standard blade refurbishment process to restore the overall blade length, including the rail around the tip cap pocket.
When the blades run in the engine, the tip of the blades have a very tight clearance with the engine case to prevent gasses flowing through the engine from passing in an inefficient fashion over the tops of the blades. In most engine designs, the blades actually contact a rub surface in the engine case. There is therefore some amount of physical wear on the blade tips. If the contact is extreme, it may result in fracture or breakage of portions of the tip rail. Blade damage of this type is dependent upon the basic blade design, along with the operating conditions experienced by any individual engine.
Turbine blades that are particularly susceptible to fracture of tip rails around the blade tip cap pocket are designed and manufactured with a very tall, thin rail around the blade tip. The tip rail is typically 0.250xe2x80x3-0.300xe2x80x3 high, with a wall thickness as thin as 0.015xe2x80x3. It is common for the tip rail on these blades to fracture during engine operation. The breakage occurs on both new and previously repaired parts and regardless of the type of alloy used to restore the tip rail. Even though engines may be allowed to operate with this type of distress, there is a measurable performance loss due to lack of proper clearances at the tips of the affected blades. The performance loss translates into increased operating costs to the engine operators. There is also significant adverse cost impact when engines must be removed from service for repair of these damaged blades.
Briefly, a process is provided for repairing a blade tip cap of a turbine blade comprising restoring the tip rail of the blade tip cap and attaching a strengthening rib across the blade tip cap connecting a concave wall to a convex wall of the tip rail. Preferably, the strengthening rib is positioned over an internal rib below the blade tip cap of the turbine blade.